Development of a Two-Dimensional Saturated
Zone Flow and Transport Model for Yucca Mountain, Nevada

Saturated zone flow paths
from beneath the potential repository location are derived from
saturated zone flow modeling and used as the basis for performance
assessment transport modeling using a series of 1D streamtubes.

Statement of Problem: The client
needed a model of far-field radionuclide transport that incorporated
dispersive and sorptive processes characteristic of the saturated
fractured tuff aquifer and the alluvial aquifer near Yucca Mountain,
Nevada. The model was used to establish the technical foundations for a
new dose-based regulation for Yucca Mountain and provide a model
abstraction for use in a Total-System Performance Assessment code.

Approach and Accomplishments:
Using background information derived from a regional-scale flow modeling
study of the Nevada Test Site conducted by the U.S. Geological Survey, scientists at CNWRA® developed 2D site-scale flow and transport models for a region
extending from Yucca Mountain to an area of Amargosa Valley where
farmland is irrigated with groundwater. The steady-state flow model,
constructed with the MAGNUM-2D finite-element simulator, was calibrated
to 146 measured values of hydraulic head using an automated inverse
procedure. Radioactive decay and advective-dispersive transport of
sorbing and nonsorbing radionuclides were modeled using the CHAINT
transport simulator, which also accounts for ingrowth of radioactive
decay daughter products. Transport simulations were conducted to
estimate unretarded travel times from the potential repository to a
compliance boundary located 20 km to the south and to assess the degree
of dose reduction realized by natural attenuation during transport.

Client Benefits: Results from
simulations conducted with the transport model for Yucca Mountain were
used by the U.S. Nuclear Regulatory Commission to evaluate various dose-based
regulatory criteria for the proposed repository. A simplified 1D streamtube network model was abstracted from the saturated zone
transport model for use in the Total-system Performance Assessment
computer code.